Enthalpy-Entropy Compensation Phenomenon Observed for Different Surfactants in Aqueous Solution

Based on previously reported thermodynamic data such as changes of the Gibbs energy (DeltaG(m)( degrees )), the enthalpy (DeltaH(m)( degrees )), and the entropy (DeltaS(m)( degrees )) on micelle formation of more than 15 species of surfactants (including nonionic, anionic, and cationic surfactants), plots of DeltaH(m)( degrees ) vs DeltaS(m)( degrees ) (not of DeltaS(m)( degrees ) vs DeltaH(m)( degrees ), as is usually done) were made. For each surfactant, a linear relation having almost the same slope (1/307 K(-1)) within a small error (+/-2.3%) but a different intercept (varsigma) depending on the surfactant species was obtained, i.e., DeltaS(m)( degrees ) = (1/307)DeltaH(m)( degrees ) + varsigma, where 1/307 (K(-1)) means that the so-called compensation temperature (T(C)) is 307 K. Strictly speaking, T(C) ranges from 299 to 315 K, depending on the species. The intercept corresponds to the entropy change at a specific temperature giving DeltaH(m)( degrees ) = 0, at which the driving force of micelle formation comes only from the entropy term; this temperature is characteristic of the surfactant species. On the other hand, the compensation temperature has no significant meaning other than a mean temperature studied. Copyright 1999 Academic Press.